indispensable (adj.)

• basic
• cardinal
• central
• compelling
• compulsory
• crucial
• demand
• essential
• focal
• fundamental
• imperative
• important
• indicated
• ineluctable
• inescapable
• irreducible
• irreplaceable
• key
• life-and-death
• life-or-death
• mandatory
• material
• must
• necessary
• necessitous
• need
• needed
• needful
• obligatory
• prerequisite
• required
• requisite
• substantive
• unavoidable
• urgent
• vital
• wanted